Anal incontinence after a prolonged second stage of labor in primiparous women

The objective was to investigate the effect of delivery mode on anal incontinence 1–2 years after delivery in primiparous women with prolonged second stage of labor. This population-based cohort and questionnaire study performed in Stockholm Region, Sweden, included 1302 primiparous women with a second stage ≥ 3 h from December 1st, 2017 through November 30th, 2018. Background characteristics and outcome data were retrieved from computerized records. Questionnaires based on Wexner score were distributed 1–2 years after delivery. Risk of anal incontinence, defined as Wexner score ≥ 2, was calculated using logistic regression and presented as crude and adjusted odds ratios (OR and aOR) with 95% confidence intervals (CI). Compared with cesarean section, vacuum extraction was associated with anal incontinence (aOR 2.25, 95% CI 1.21–4.18) while spontaneous delivery was not (aOR 1.55, 95% CI 0.85–2.84). Anal incontinence was independently associated with obstetric anal sphincter injuries (aOR 2.03, 95% CI 1.17–3.5) and 2nd degree perineal tears (aOR 1.36, 95% CI 1.03–1.81) compared with no or 1st degree perineal tear. Obstetric anal sphincter injury at vacuum extraction inferred the highest risk of anal incontinence (aOR 4.06, 95% CI 1.80–9.14), compared with cesarean section. Increasing duration of the prolonged second stage did not affect the risk.


Material and methods
Study population and exposure. The source population was all women who gave birth in any of the six labor ward hospitals in Stockholm, Sweden, from December 1st, 2017 through November 30th, 2018. The deliveries in Stockholm constitute approximately 25% of all deliveries in Sweden 31 . All hospitals have similar facilities and staffing and use the same computerized medical records system (Obstetrix, Cerner Corporation, Sweden) with the same evaluation standards supported by documentation templates and compulsory checkboxes. Virtually all deliveries take place in the hospitals and are attended by midwives, with residents and consultants available at all times. All hospitals have operating facilities that are immediately available. All obstetric care is tax funded and free of charge. The medical records cover antenatal care, labor and delivery, and postnatal care for all births in Stockholm. At the time of data collection there was neither a regional nor national guidline regarding management of the second stage. The common practice in Sweden is to await the woman's spontaneous urge to push. Oxytocin is administered after 1-2 h without progress in the passive phase of second stage or after 30 min of inefficient pushing. Active pushing is encouraged after 3 h without progress in the passive phase.
Inclusion criteria in the study were primiparous women with a term (≥ 37 gestational weeks), live, single, cephalic birth, and a prolonged second stage of labor ≥ 3 h. Maternal and neonatal characteristics and outcomes were retrieved from computerized medical records by the Stockholm Region Enterprise Data Warehouse. We chose a 3-h duration cut-off since a shorter duration is considered normal [1][2][3][4]8,32 , and thereby the duration in itself is not considered an indication for intervention before 3 h.
During the stated time-period there were 12,058 primiparous women with a term, live, single, cephalic birth. We excluded women with an elective cesarean section (n = 1030). Second stage of labor duration was calculated from two time points in the partograph: time of birth and the timepoint of first notation of full cervical dilation. We excluded women who had unknown cervical dilation and those with full cervical dilation for < 3 h (Fig. 1). The remaining 3042 women met the inclusion criteria. For these women, we retrieved contact information, information on delivery mode, and second stage duration. Women with missing contact information (n = 190), duplicate registrations (n = 2), forceps delivery (n = 2), and outliers with a second stage exceeding 13 h (n = 3) were not further analyzed (Fig. 1).
Nulliparous women with live, singleton, cephalic birth at ≥37+0 gestational weeks in Stockholm region 171201-181130, n=12058 Delivery before full cervical dilation or unknown cervical dilation, n=1527 Nulliparous women with live, singleton, cephalic birth at ≥37+0 gestational weeks and full cervical dilation ≥3:00 h, n=3042 Full cervical dilation <3:00 h, n=6459 Elective cesarean section, n=1030 Trial of labor, n=11028 Full cervical dilation, n=9501 Second stage ≥13 hours, n=3 Eligible study participants, n=2845 Letter in return, address unknown, n=76 No response, n=1466 Responses, n=1303 Did not meet the inclusion criteria, n=1 Final study cohort, n=1302 Spontaneous vaginal delivery, n=923 Vacuum extraction, n=280 Cesarean section, n=99 Forceps delivery, n=2 Missing contact information, n=190 Duplicates, n=2  www.nature.com/scientificreports/ In total, 2845 women were eligible study participants (Fig. 1). They were sent a letter containing study information, a consent form, and a questionnaire on pelvic floor function 33 . One postal reminder was sent after 2 weeks. Letters were sent and received from December 1st, 2019 through June 30th, 2020 ensuring a time interval of 1-2 years after delivery.
Main exposure was mode of delivery after a prolonged second stage (≥ 3 h): spontaneous vaginal delivery, vacuum extraction, and cesarean section. Secondary exposures were duration of second stage, duration of fetal head station below the ischial spines, and perineal injury.
Outcome measures. The questionnaire used in the study is based on the 1-year follow-up questionnaire developed and used in the Swedish Perineal Tear Register, containing questions about urinary incontinence, anal incontinence, symptoms of pelvic organ prolapse, and sexual function 33 . Anal incontinence is measured by Wexner score 34 , containing the following parameters: incontinence of flatus, liquid or solid stool, the use of pads, and alteration of lifestyle. Each parameter is assessed with a scoring system (never = 0, rarely = 1, sometimes = 2, usually = 3, and always = 4). Maximum score is 20 34 . Wexner score is the most cited anal incontinence score 35 . It was developed as an interview-based score but has been validated also as a self-reporting scoring system with highly consistent results compared to the results obtained by interview 36 . Jangö et al. 37 evaluated Wexner score in women with previous OASIS and found Wexner score ≥ 2 to be a significant cut-off for affected quality of life. Thus, we chose Wexner score ≥ 2 as our primary outcome. Women with a subsequent delivery were instructed to answer the questions based on symptoms before the second pregnancy.
Episiotomy, perineal and vaginal tears were classified according to the International Classification of Diseases-10 (ICD-10) Swedish Edition by clincical examination. We collected information on degree of injury from diagnosis codes and procedure codes. We categorized women into three groups according to their most severe injury: 1st degree perineal tear (O70.0), including women with no diagnosis and women with an isolated vaginal tear (O71.4), 2nd degree perineal tear (O70.1) also including women with only an episiotomy diagnosis (TMA00), and OASIS including 3rd (O70.2) and 4th degree perineal tears (O70.3).
Maternal characteristics, delivery characteristics, and outcomes in women with Wexner score ≥ 2 compared with < 2 were calculated and compared by Chi 2 test. Median Wexner score was calculated by Kruskal-Wallis test. A p value < 0.05 was considered significant. Relative risk of Wexner score ≥ 2 compared with < 2 was calculated using complete case analysis with unconditional univariate and multivariate logistic regression with cesarean section as the reference for the main exposure 'mode of delivery' . Results are presented as crude and adjusted odds ratios (OR and aOR) with 95% confidence intervals (CI). We used directed acyclic graphs (DAG) to select possible covariates for the adjusted analyses. Different DAGs were created for the main exposure 'mode of delivery' and for the secondary exposures, 'duration of second stage/duration of fetal station below the ischial spines' , 'perineal injury' , and the outcome Wexner score ≥ 2 ( Supplementary Fig. S1). We included two different models for adjustment. In model A, adjustments were made for the potential confounders, but not the mediator pathway. In model B adjustements were made for the potential confounders and for the mediator pathway (Supplementary Fig. S1). The potential interaction between delivery mode and perineal injury on Wexner score ≥ 2 was calculated and presented as prevalence, proportions, and unadjusted odds ratios using univariate logistic regression with the terms 'delivery mode' , 'perineal injury' and the interaction term 'delivery mode*perineal injury' . Cesarean section was used as reference.
Missing data is presented in the tables when prevalent. Missing data on duration of fetal station below the ischial spines was categorized as "unknown" and missing data on perineal or vaginal tears was categorized as "no diagnosis", to avoid exclusion in regression analyses. Missing data in other variables constituted a minor proportion (< 3%) except for BMI (4.2%). We performed sensitivity analyses with multiple imputation of the covariate BMI with no difference compared with the presented results (data not presented). We also performed sensitivity analyses excluding women with a subsequent delivery during the follow-up period, and women who answered the questionnaire later than 24 months after delivery. No difference was found compared with the presented results (data not presented). Therefore all women were included.

Results
We received 1302 responses of 2845 distributed questionnaires (response rate 45.8%), while 76 questionnaires were returned to sender due to unknown address. One woman had been included despite a breech presentation and was excluded before analyses (Fig. 1). The final study cohort consisted of 1302 primiparous women with a live, single cephalic birth at ≥ 37 gestational weeks and a second stage duration ≥ 3 h. Of these, 923 (70.9%) women had a spontaneous vaginal delivery, 280 (21.5%) a vacuum extraction, and 99 (7.6%) a cesarean section (Fig. 1). OASIS was diagnosed in 37 (4.0%) women with spontaneous vaginal delivery and in 35 (12.5%) women with vacuum extraction. This is similar to the national average for primiparous women 31 . Women with Wexner score ≥ 2 were older than women with Wexner score < 2, but otherwise similar ( Table 1). The prevalence of Wexner score ≥ 2 was highest after vacuum extraction, followed by spontaneous vaginal delivery, and lowest after cesarean section (Fig. 2). Wexner score medians and ranges were significantly higher in both vaginal modes of delivery compared with cesarean section (Fig. 2). Characteristics and outcomes according to mode of delivery can be found as Supplementary Table S1 and S2.
In women with vaginal delivery, the prevalence of Wexner score ≥ 2 successively increased with degree of perineal injury (Table 2). Women with episiotomy only had the lowest prevalence of Wexner score ≥ 2 ( Table 2). Neither second stage duration, duration of fetal station below the ischial spines, fetal position, birthweight, nor head circumference affected the prevalence of Wexner score ≥ 2 ( Table 2). When collapsing durations over 5 h into one group, no significant association was seen (data not presented).
Vacuum extraction was associated with Wexner score ≥ 2, also after adjusting for perineal injury (Table 3). Spontaneous vaginal delivery was not associated with a Wexner score ≥ 2 after adjustments ( Table 3). The risk of Wexner score ≥ 2 was increased in women with OASIS and women with a 2nd degree tear or episiotomy compared with women with no or 1st degree perineal injury (Table 3).
There was an interaction between mode of delivery and perineal injury on the risk of Wexner score ≥ 2 ( Table 4). In women with no or 1st degree perineal injury, the risk of Wexner score ≥ 2 was not increased compared with cesarean section. In women with 2nd degree injury or episiotomy the risk of Wexner score ≥ 2 was increased after both modes of vaginal delivery (Table 4). Women with an OASIS at vacuum extraction had a fourfold risk of Wexner score ≥ 2, while this risk increase did not reach statistical significance in women with OASIS in spontaneous vaginal delivery (Table 4).
In total, 147 (11.3%) women had a subsequent delivery during the follow-up period. In the sensitivity analyses, there was no difference in prevalence of Wexner score ≥ 2 between women without a subsequent delivery and the total study cohort (data not presented). Of all 1302 answers, 69 women (5.3%) answered the questionnaire later (25-27 months) than the stipulated 12-24 months after delivery. There was no difference in prevalence of Wexner score ≥ 2 between women with a time since delivery 12-24 months and the total study cohort (data not presented).

Discussion
Main findings. This study showed that vacuum extraction but not spontaneous vaginal delivery, compared with cesarean section, increased the risk of anal incontinence 1-2 years after a first delivery with a prolonged second stage, irrespective of degree of perineal injury. We could not show that the duration of prolonged second stage, nor the duration of fetal station below the ischial spines, affected the risk of anal incontinence. In women with vaginal delivery, a perineal injury especially OASIS, was the most important risk factor for anal incontinence. The combination of vacuum extraction and OASIS was the most detrimental to anal continence, without any relation to increasing duration of the prolonged second stage.
Interpretation. Few other studies have had a comparable approach, differing in exposure or outcome, or are associated with a considerable power problem 29,39,40 . When comparing cesarean section in the second stage to spontaneous vaginal delivery, no difference in long-term anal incontinence has been found 41 . When comparing operative vaginal delivery to spontaneous vaginal delivery after a prolonged second stage, long-term anal incontinence has been found to be similar or increased, especially by OASIS, but not a prolonged second stage 30,42 . Thus, most existing observations support our finding that second stage duration in itself does not affect the risk of anal incontinence, but mode of delivery and its feared consequences, especially OASIS, do.
Apart from when to end a prolonged second stage, the duration of second stage is dependent on the three classic "Ps of labor": "the pelvis, passenger, and power" 43 . Power includes the modifiable factors contractions and maternal pushing. The use of oxytocin and immediate pushing will significantly shorten the second stage 44,45 . The timing of pushing does however not influence the risk of OASIS 44,45 , or fecal incontinence 46 .

Clinical implications and further research. Several factors influence the decision on when and how
to intervene. The station of the fetal head is essential for the choice between instrumental vaginal delivery or cesarean section, but also assessement of maternal and fetal risk factors for unsuccessful instrumental vaginal delivery followed by a potentially difficult cesarean section. Risk of OASIS, shoulder dystocia, anesthesiological, www.nature.com/scientificreports/ or surgical complications also influence the desicion. To this, the contractions, maternal pushing efforts, signs of fetal distress, parents' wish, and the physician's preferred mode of delivery can be added. Thus the best way forward in a prolonged second stage is a complex jigsaw, to which the results of our study may provide one piece to inform management: The wait and see approach does not in itself increase the risk of anal incontinence.
Our results indicate that the forced passage by operative vaginal delivery through the pelvic floor is the critical event. If a spontaneous vaginal delivery seems possible, non-instrumental management and care to avoid OASIS should be the recommendation. If vacuum extraction is chosen, maximum efforts should be made to avoid OASIS. With regard to protective measures against OASIS which is known to be the most important risk factor of anal incontinence in women 27,28 , it is noteworthy that women with episiotomy had no increased risk of www.nature.com/scientificreports/ anal incontinence. This supports liberal or even routine use of episiotomy in vacuum extraction in nulliparous women, since a lateral or mediolateral episiotomy in this specified population may half the risk of OASIS 47 .
Whether routine use of episiotomy in vacuum extraction in nulliparous women will decrease long-term anal continence remains to be assessed in an ongoing randomized controlled trial 48 . It has been suggested that levator ani muscle injury contributes to anal incontinence 49 . Prolonged second stage, operative vaginal delivery, and OASIS are associated with levator ani muscle injury 50,51 . At the time of data collection, there was no separate ICD-10 diagnosis code for levator ani muscle injury, and such occult injuries could be an unmeasured confounder. Since January 1st 2020 the Swedish ICD-10 has been updated with specific codes for levator ani muscle injuries which enables further research on the association between these injuries and anal incontinence.
When spontaneous vaginal delivery is no longer an option and vacuum extraction seems unfavorable, cesarean section could be the preferable mode of delivery to prevent future anal incontinence. To test if cesarean section is preferable to vacuum extraction in a prolonged second stage to prevent future anal incontinence, a randomized controlled trial could be conducted. However, it could be a challenge to complete such a trial.

Strengths and limitations.
To our knowledge, this is the largest study investigating the long-term risk of anal incontinence depending on mode of delivery after a prolonged second stage of labor. Our method of extracting data from computerized medical records enabled a large set of variables and duration details which are rarely possible using register data. We used the validated Wexner score to quantify anal incontinence and applied a cut-off developed for a similar population 37 . The time-point of measurement, 12-24 months after delivery, represents a time-point after recovery from childbirth which should reflect symptoms that are likely to remain 52 .
The major limitation was the moderate response rate, entailing a possible self-selection bias. Women with symptoms could be more likely to respond, causing an overestimate of the overall prevalence of anal incontinence. We assessed available data on non-respondents and found no difference compared to respondents in second stage duration or proportion of vacuum extractions, but a slightly smaller proportion of spontaneous vaginal deliveries and larger proportion of cesarean sections. Another important limitation is the risk of lack of power. Some observed associations were not statistically significant, specifically between Wexner score ≥ 2 and an increasing second stage duration, with a decreasing number of women in each duration interval. Albeit, there was no difference if durations longer than 5 h were collapsed into one interval. Thus, we cannot exclude an association between anal incontinence and very long second stage durations. That said, we considered 1-year data to be sufficient to explore if there were clinically important differences.
Another limitation was that the questionnaire was only available in Swedish. This could result in an availability bias due to language barrier. This was reflected in a somewhat lower response rate in women delivered in hospitals with a higher proportion of women with another primary language. Also, one limitation was that other variables that potentially could affect the results such as maternal exercise habits, education, or occupation was not available or poorly reported in the data source and hence could not be studied.